Biological and human clinical research has shown utility of electrical nerve stimulation for urinary incontinence and a broad group of urological disorders. This invention is directed to the adjunct therapy for these disorders utilizing an implanted lead-receiver and an external stimulator with predetermined stimulation programs.
In considering the background of urinary urge incontinence, FIG. 1 shows a sagittal section of the human female pelvis showing the bladder 10 and urethra 13 in relation to other anatomic structures. Urinary continence requires a relaxed bladder during the collecting phase and permanent closure of the urethra, whereas at micturition (urination), an intravesical pressure above the opening pressure of the simultaneously relaxing urethra has to be generated. These functions of the bladder and urethra are centrally coordinated and non-separable. At bladder filling, the sensation of urge is mediated by slowly adapting mechanoreceptors in the bladder wall and the same receptors provide the triggering signal for micturition and the main driving force for a sustained micturition contraction. The mechanoreceptors are, technically speaking, tension receptors. It has been found that they respond equally well to tension increases induced passively by bladder filling and those induced actively by a detrusor contraction. These receptors have high dynamic sensitivity and are easily activated by external pressure transients, as may occur during coughing or tapping of the abdominal wall. Their faithful response to active changes in bladder pressure is well illustrated.
When sufficiently activated, the mechanorecptors trigger a coordinated micturition reflex via a center in the upper pons 88, as depicted schematically in FIG. 2. The reflex detrusor 92 (muscle in the wall of the urinary bladder) contraction generates an increased bladder pressure and an even stronger activation of the mechanoreceptors. Their activity in turn reinforces the pelvic motor output to the bladder, which leads to a further increase in pressure and more receptor activation and so on. In this way, the detrusor contraction is to a large extent self generating once initiated. Such a control mechanism usually is referred to as a positive feedback, and it may explain the typical all-or-nothing behavior of the parasympathetic motor output to the bladder. Once urine enters the urethra, the contraction is further enhanced by reflex excitation from urethral receptors. Quantitatively, the bladder receptors are most important.
A great advantage of the positive feedback system is that it ascertains a complete emptying of the bladder during micturition. As long as there is any fluid left in the lumen, the intravesical pressure will be maintained above the threshold for the mechanoreceptors and thus provide a continuous driving force for the detrusor. A drawback with this system is that it can easily become unstable. Any stimulus that elicits a small burst of impulses in mechanoreceptor afferents may trigger a full-blown micturition reflex. To prevent this from happening during the filling phase, the neuronal system controlling the bladder is equipped with several safety devices both at the spinal and supraspinal levels.
The best-known spinal mechanism is the reflex control of the striated urethral sphincter 90, which increases its activity in response to bladder mechanoreceptor activation during filling. An analogous mechanism is Edvardsen's reflex, which involves machanoreceptor activation of inhibitory sympathetic neurons to the bladder. The sympathetic efferents have a dual inhibitory effect, acting both at the postganglionic neurons in the vesical ganglia and directly on the detrusor muscle of the bladder 92. The sphincter and sympathetic reflexes are automatically turned off at the spinal cord level during a normal micturition. At the supraspinal level, there are inhibitory connections from the cerebral cortex and hypothalamus to the pontine micturition center. The pathways are involved in the voluntary control of continence. Other inhibitory systems seem to originate from the pontine and medullary parts of the brainstem with at least partly descending connections.
Bladder over-activity and urinary urge incontinence may result from an imbalance between the excitatory positive feedback system of the bladder 10 and inhibitory control systems causing a hyperexcitable voiding reflex. Such an imbalance may occur after macroscopic lesions at many sites in the nervous system or after minor functional disturbances of the excitatory or inhibitory circuits. Urge incontinence due to detrusor instability seldom disappears spontaneoulsly. The symptomatic pattern also usually is consistent over long periods.
Based on clinical experience, subtypes of urinary incontinence include, Phasic detrusor instability and uninhibited overactive bladder. Phasic detrusor instability is characterized by normal or increased bladder sensation, phasic bladder contractions occurring spontaneously during bladder filling or on provocation, such as by rapid filling, coughing, or jumping. This condition results from a minor imbalance between the bladder's positive-feedback system and the spinal inhibitory mechanisms. Uninhibited overactive bladder is characterized by loss of voluntary control of micturition and impairment of bladder sensation. The first sensation of filling is experienced at a normal or lowered volume and is almost immediately followed by involuntary micturition. The patient does not experience a desire to void until she/he is already voiding with a sustained detrusor contraction and a concomitant relaxation of the urethra, i.e., a well-coordinated micturition reflex. At this stage, she/he is unable to interrupt micturition voluntarily. The sensory disturbance of these subjects is not in the periphery, at the level of bladder mechanoreceptors, as the micturition reflex occurs at normal or even small bladder volumes. More likely, the suprapontine sensory projection to the cortex is affected. Such a site is consistent with the coordinated micturition and the lack of voluntary control. The uninhibited overactive bladder is present in neurogenic dysfunction.
Since bladder over-activity results from defective central inhibition, it seems logical to improve the situation by reinforcing some other inhibitory system. Patients with stress and urge incontinence are difficult to treat adequately. Successful therapy of the urge component does not influence the stress incontinence. While an operation for stress incontinence sometimes results in deterioration of urgency. Electro stimulation is a logical alternative in mixed stress and urge incontinence, since the method improves urethral closure as well as bladder control. Drug treatment often is insufficient and, even when effective, does not lead to restoration of a normal micturition pattern.
Neuromodulation is a technique that applies electrical stimulation to the sacral nerves, (a general diagram of spinal cord and sacral nerves 85 is shown in FIG. 3). The aim of this treatment modality is to achieve detrusor 92 inhibition by chronic electrical stimulation of afferent somatic sacral nerve fibers 85 via implanted electrodes coupled to a subcutaneously placed pulse generation means.
The rationale of this treatment modality is based on the existence of spinal inhibitory systems that are capable of interrupting a detrusor 92 contraction. Inhibition can be achieved by electrical stimulation of afferent anorectal branches of the pelvic nerve, afferent sensory fibers in the pudendal nerve and muscle afferents from the limbs. Most of these branches and fibers reach the spinal cord via the dorsal roots of the sacral nerves 85. Of the sacral nerve roots the S3 root is the most practical for use in chronic electrical stimulation.
Most nerves in the human body are composed of thousands of fibers, of different sizes designated by groups A, B and C, which carry signals to and from the brain. A peripheral nerve, for example, may have approximately 100,000 fibers of the three different types, each carrying signals. Each axon or fiber of that nerve conducts only in one direction, in normal circumstances. The A and B fibers are myelinated (i.e., have a myelin sheath, constituting a substance largely composed of fat, whereas the C fibers are unmyelinated.
A commonly used nomenclature for peripheral nerve fibers, using Roman and Greek letters, is given in the table below:
ExternalConductionDiameterVelocityGroup(μm)(m/sec)Myelinated FibersAα or IA12–20 70–120Aβ:IB10–1560–80II 5–1530–80Aγ3–815–40Aδ or III3–810–30B1–3 5–15Unmyelinted fibersC or IV0.2–1.50.5–2.5The diameters of group A and group B fibers include the thickness of the myelin sheaths. Group A is further subdivided into alpha, beta, gamma, and delta fibers in decreasing order of size. There is some overlapping of the diameters of the A, B, and C groups because physiological properties, especially the form of the action potential, are taken into consideration when defining the groups. The smallest fibers (group C) are unmyelinated and have the slowest conduction rate, whereas the myelinated fibers of group B and group A exhibit rates of conduction that progressively increase with diameter. Group B fibers are not present in the nerves of the limbs; they occur in white rami and some cranial nerves.
Compared to unmyelinated fibers, myelinated fibers are typically larger, conduct faster, have very low stimulation thresholds, and exhibit a particular strength-duration curve or respond to a specific pulse width versus amplitude for stimulation. The A and B fibers can be stimulated with relatively narrow pulse widths, from 50 to 200 microseconds (μs), for example. The A fiber conducts slightly faster than the B fiber and has a slightly lower threshold. The C fibers are very small, conduct electrical signals very slowly, and have high stimulation thresholds typically requiring a wider pulse width (300–1,000 μs) and a higher amplitude for activation. Selective stimulation of only A and B fibers is readily accomplished. The requirement of a larger and wider pulse to stimulate the C fibers, however, makes selective stimulation of only C fibers, to the exclusion of the A and B fibers, virtually unachievable inasmuch as the large signal will tend to activate the A and B fibers to some extent as well.
In neuromodulation, the entire innervation system should be intact. As shown schematically in FIG. 5, the procedure consists of placing electrodes 61,62 in one of the sacral foraman as close to the pelvic plexus and pudendal nerve as possible and connecting the lead 59 with a means for electrical stimulation 49. The hypothesis behind neuromodulation of the sacral roots (sensory and motor) is to correct, by the use of regulating electrical impulses, the dys-synergic activities of the cholinergic, adrenergic, and motor reflex pathways that initiate vesical storage and micturition. Although some theories have been developed that explain the effects of neuromodulation, most of the results are based on empiric findings in human studies. Some animal experiments and electrophysiologic studies in humans show there is a spinal inhibitory action through the afferent branches of the pelvic and pudendal nerves. It is not clear whether neuromodulation primarily influences the micturiction center located near the thalamus in the brain. Some maintain that there is a direct correction of the dys-synergis of the pelvic floor (pudendal nerve) by influencing the abnormal contractility of the pelvic floor.
A neurophysiological explanation for the effectiveness of this treatment modality in detrusor instability is based on animal experiments and electrophysiological studies in humans. Electrical stimulation for the treatment of urinary incontinence has evolved over the past 40 years. The mechanism of action of electrical stimulation was investigated initially in animal models. Over 100 years ago, Griffiths demonstrated relaxation of a contracted detrusor during stimulation of the proximal pudendal nerve in the cat model and further work clarified the role of pudendal afferents in relation of the detrusor. Spinal inhibitory systems capable of interrupting a detrusor contraction can be activated by electrical stimulation of afferent anorectal branhes of the pelvic nerve, afferent sensory fibers in the pudendal nerve and muscle afferents from the limbs. The effectiveness of neuromodulation in humans has been objectively demonstrated by urodynamic improvement, especially in light of the fact that such effects have not been noted in drug trials.
Neuromodulation also acts on neural reflexes but does so internally by stimulation of the sacral nerves 85. Sacral nerve 85 stimulation is based on research dedicated to the understanding of the voiding reflex as well as the role and influence of the sacral nerves 85 on voiding behavior. This research led to the development of a technique to modulate dysfunctional voiding behavior through sacral nerve stimulation. It is thought that sacral nerve stimulation induces reflex mediated inhibitory effects on the detrusor through afferent and/or efferent stimulation of the sacral nerves 85.
Even though the precise mechanism of action of electrical stimulation in humans is not fully understood, it has been shown that sensory input traveling through the pudendal nerve can inhibit detrusor activity in humans. Most experts believe that non-implanted electrical stimulation works by stimulating the pudendal nerve afferents, with the efferent outflow causing contraction of the striated pelvic musculature. There is also inhibition of inappropriate detrusor activity, though the afferent mechanism has yet to be clarified. There is consensus that the striated musculature action is able to provide detrusor inhibiton in this setting, though data supporting this hypotheses are lacking.
In summary, the rationale for neuromodulation in the management of such patients is the observation that stimulation of the sacral nerves via electrical stimulation can inhibit inappropriate neural reflex behavior.